In an electrical generating plant powered by a steam turbine, the high pressure stage of the turbine is constructed to receive steam through a plurality of arcuately spaced nozzles adjacent the turbine first stage or impulse blading. The steam then flows from the impulse blading to an impulse chamber through the remaining rows of high pressure blades. The nozzles are segregated into individual groups about the circumference of the impulse blading; and an individual governor valve controls the steam flow through each nozzle group, the number of which may vary for respective valves. There are, typically eight governor valves in a fossil-fuel powered generating plant, and four governor valves in a nuclear powered generating plant. Steam is directed to the governor valves through one or more throttle or stop valves from the steam source.
When starting up the turbine, it is common practice to operate all the governor valves as a single valve to admit the steam in a full 360 degree arc through the nozzles to the inpulse blading. This practice, which is termed single valve, or full arc, operation permits the heating of all blading evenly which minimizes thermal shock. However, when the turbine is "hot" and all the valves are admitting the required steam, in a partially open position, the efficiency of the plant is considerably reduced because of the pressure drop or throttling action across all the partially open valves. In this situation, the efficiency of the turbine can be increased by admitting the steam through a portion of so-called partial arc of fully-open valves with the steam flow variations being controlled by one or more of the remaining valves in a sequential manner.
In the automatic control of governor valves, in either the single or sequential valve mode of operation, a load demand or in turn a flow demand signal may be used to control the position of the individual valves. When the valve of such representation bears a definite relationship to the position or change of position of the valves, such control may be termed feedforward. This is in contrast to a feedback control where the value of an error representation is related to the distance a valve is required to travel, for example, from its instant position in order to null or change the representation to a predetermined value. In any event, it is necessary to have an accurate relationship between steam flow and valve lift position for accurate turbine control.
For a valve operating in conjunction with a group of nozzles, the position/flow relationship remains linear as long as there is critical flow across the valve. However, the valve characteristic begins to change, as soon as the pressure drop across the nozzles, is reduced below critical, or in other words, when the valve is subjected to the effect of pressure drop across the nozzles or of impulse chamber pressure, the maximum flow through the valve when fully open is reduced. A change in inlet steam pressure also affects the steam flow; and a steam flow demand change that results from a substantial change in pressure, can produce increased oscillation between the boiler and turbine control system.
Heretofore, in proposed systems, as far as known, the characterization of lift position versus steam flow was fixed for a given load demand at a predetermined pressure. Thus, of course, at such flow and pressure, accuracy between flow and valve position could be presumed, but for different load or flow levels and at other pressures, such accuracy is not obtainable.